Yeah, you'd have to bead blast the rust and then coat it with paint or powder coat. Ceramic coating (done properly) is quite expensive.

I would advise against just covering it w/ a turbo blanket but I've also seen plenty of rusty manifolds that were operating just fine. I'd coat it if it were me.

Yes, I forgot to mention, It's quite costly, but it will also help with heat. If you did get it coated and also put a blanket over it would yield you the best results. If you did nothing and just left the rusty manifold, youd probably be fine as well. I would coat and cover.